The disclosed technology regards a durable, fire resistant, energy absorbing and cost-effective strengthening system, useful especially at high stress concentration zones of structural joints and members, and adjoining other connections and re-entrant angles of members, applicable for both in-service structures and new construction. The system is ideally suited to strengthen joints and connections and structural members/components with ledges and re-entrant angles which receive multiple other structural components under multiple load paths, including dynamic load paths resulting from high winds, explosive blasts and earthquakes. Applications include bridge structures, roof trusses, openings and ledges in walls and slabs of buildings, bridges, lattice towers, truss joints and other infrastructure systems, as well as planes, ships and other complex structural systems.
Over the past twenty years, increases in traffic flow and vehicle weight, environmental pollution, application of de-icing agents, low-quality and aged structural materials including expansion joints and waterproofing membranes, and insufficient/inadequate design, maintenance and rehabilitation approaches, have led to the rapid deterioration of bridges and other structures. Repair of these structures to preserve the structure and safeguard human life are becoming a serious technical and costly problem in many countries.
Advanced composites of high grade fibers and fabrics with binders such as thermosets and thermoplastics are beginning to play a significant role in construction applications, particularly in strengthening and rehabilitating existing bridges that have deteriorated due to their age and environmental influences. Current systems of joint repair include haphazardly bonding discontinuous fiber reinforced polymer (FRP) sheets at the re-entrant corners of a joint. FRP laminates are composite materials built from a combination of sheets made from carbon, glass or aramid fibers bonded together with a polymer matrix, such as epoxy, polyester or vinyl ester. As currently used, FRP can be applied to strengthen beams, columns and slabs of building and bridge structural elements and other structural components/members, and can increase the strength of structural members even after they have been severely damaged due to loading or other conditions. Further, application of FRP sheets in this haphazard manner has become a cost-effective material in a number of field applications strengthening concrete, masonry, steel, cast iron and timber structures, and is frequently used to retrofit structures in civil engineering.
When used to strengthen joints and structural components, multiple sheets/strips of FRP are wrapped about a joint, using epoxy or other adhesives; these sheets are typically applied in a haphazard-manner, without utilizing the material's ability to greatly absorb shocks and minimize stress concentration around a junction, and without maximizing the rupture stress resistance of the materials through confinement and damping. Therefore there remains a serious concern in the industry as to the long-term integrity and likelihood of cyclic fatigue loading on joints and components bonded in this manner. Other concerns include application errors, such as improper curing of the resins, moisture absorption and ultraviolet light exposure of the FRP composites that may affect strength and stiffness. For example, certain resin systems in glass fiber composites, are found ineffective in the presence of moisture. These issues could lead to de-bonding or delamination of the FRP sheets from the substrate, as well as shear failure due to inadequate confinement of the core joint.
Furthermore, prior art methods of randomly applying FRP composite sheets about a joint without focusing on minimizing stresses frequently result in lopsided strengthening of the joint, rather than uniformly minimizing stress concentrations (including axial, bending, shear and torsion stresses or their combinations). Similarly, prior art methods include discrete anchoring of steel angles or plates at re-entrant corners after bonding the FRP sheets to the substrate, which lead to stress raisers including stress-corrosion, and eventually to potential delamination between the FRP and the substrate, and even cracking in the member at the long-edge of an angle. Likewise, some prior art methods place a steel angle with sharp edges at the joint, and then wrap the angle with FRP, which leads to cracking at the sharp edges. These steel angle methods lead to premature failure in the fabric due to high stress concentration and the sharp edges of the steel angle, and also stiffness mismatch between a steel angle and its substrate. Engineers have also attempted methods of welding one or more thin steel plates to a steel angle and placing it at the corners of a joint, which leads to local buckling of the web or fracture of the weld. Many classical failure modes at joints have been delayed, using current state of the art, by only small increases in mechanical properties including energy absorption; however, the above-identified limitations in the current state of the art lead to even more dramatic failures under dynamic, shock and environmental loads.
Use of the system of the disclosed technology overcomes these limitations of the prior art. The system of the disclosed technology and installation thereof in accordance with the methods hereinafter described minimizes the stress concentration effects at the re-entrant angles and may provide confinement to the joint-core. This enhances the strength, stiffness, ductility and energy absorption capacity of a joint, while minimizing stress concentration and structural and material deterioration from environmental and fire exposure. Preliminary test results indicate a significant increase in the strength, ductility and energy absorption of the joint.
Furthermore, the system allows non-intrusive, in-situ installation, and in some cases components thereof may also be designed and manufactured in-situ.